The present invention relates to an arrangement of an FM demodulator to demodulate a frequency modulated signal.
In the television broadcast using a geostationary satellite, a frequency modulated (FM) television signal is used. In a receiver for receiving this broadcast, the television signal is demodulated from a weak radio wave which is transmitted from the satellite; therefore, a PLL (phase locked loop) FM demodulator having excellent characteristic for weak electric fields is often used. FIG. 1 shows an arrangement of an ordinary PLL-FM demodulator. A frequency modulated signal which was inputted through an input terminal 1 is demodulated by a PLL-FM demodulator consisting of the closed loop which is constituted by a phase detector 2, an amplifier 3, a loop filter 4, and a voltage controlled oscillator (VCO) 5. An input line to the voltage controlled oscillator 5 is branched, to pick up the demodulated television signal from an output terminal 6. In this FM demodulator, the beat component which is generated from the phase detector 2 and which has the frequency corresponding to the difference between the input signal from the terminal 1 and the signal from the voltage controlled oscillator 5 is amplified by the amplifier 3; the phase and gain are adjusted by the loop filter 4; thereafter, the output signal from the loop filter 4 is applied to an oscillation frequency control terminal of the voltage controlled oscillator 5. The television signal as the original signal of the input FM signal is generated at the oscillation frequency control input terminal of the voltage controlled oscillator 5 due to the function of the closed loop which sets the frequency of the beat component to 0, and the closed loop serves as the FM demodulator. Therefore, the demodulation band of this FM demodulator is determined by the gain and phase characteristics which are formed by the amplifier 3 and loop filter 4 in the closed loop and can be set to the minimum necessary band; therefore, the demodulation characteristic when the electric field is weak is excellent.
FIG. 2 shows a demodulation band characteristic of the ordinary PLL-FM demodulator shown in FIG. 1. In case of satisfying the conditions such that the sum of the phase error between the input signal and a reference frequency f.sub.0 of the voltage controlled oscillator 5 and the phase error in the closed loop is .+-.90.degree. or less and that the gain of the loop is greater than 1, the PLL circuit shown in FIG. 1 is synchronized, so that the oscillation frequency from the voltage controlled oscillator 5 is shifted in accordance with the input signal frequency. Therefore, as an output of the FM demodulator, the voltage applied to the voltage controlled oscillator 5 and the DC voltage proportional to the difference between the reference frequency f.sub.0 and the frequency of the input signal along characteristic line 7 of the oscillation frequency are generated, so that the function as the FM demodulator is obtained. The frequency width in this synchronous region corresponds to the frequency demodulation band. From the above-mentioned loop conditions of the PLL, it will be appreciated that the phase error in the closed loop becomes small with an increase in loop gain, so that the frequency demodulation band becomes wide. In addition, even in the case where the gains are the same, the frequency demodulation band becomes wide as the phase distortion in the closed loop becomes small.
On the other hand, as the characteristic which is inherent to the FM demodulator, there is a threshold characteristic whereby a demodulation S/N (signal-to-noise) ratio suddenly deteriorates as a C/N (carrier-to-noise) ratio of the input FM signal becomes low. This threshold is determined by the amount of noises to be demodulated. As the amount of noises to be demodulated is less, that is, as the band width of the demodulation noises is narrow, the threshold level moves to a lower C/N ratio. In order to make the band width of the demodulation noises narrow, it is necessary to set the gain characteristic of the PLL to a narrow band.
As described above, it is desirable for the PLL-FM demodulator to have the characteristic such that the gain is large and the demodulation band is wide at the video frequency and the gain suddenly becomes small and the demodulation band is narrow at a frequency higher than the video frequency for improvement in the threshold value.
The gain characteristic of the closed loop of the PLL-FM demodulator shown in FIG. 1 is determined by the loop filter 4. In case of using a lag-lead filter having a small phase distortion as the loop filter 4, a gentle attenuation characteristic having an attenuation ratio of 6 dB at the double frequency is shown. Therefore, when the demodulation band is ensured at the video frequency in case of demodulating the FM signal having a large frequency deviation due to the video signal, there is a drawback such that the noise band becomes wide and the threshold level deteriorates. On the other hand there is another drawback such that when the threshold level is preferably set, the demodulation band at the video frequency becomes narrow, so that the demodulation cannot be performed. When another narrow band filter than the lag-lead filter is used as the loop filter 4, the phase distortion in the closed loop increases, so that even if the gain is increased, the demodulation band will not become wide.